With the recent progress of semiconductor microelectronics, a large number of less expensive wireless communication devices have been constantly provided, and equipment for wireless communications has been widely used in daily life. The era is approaching in which it will be common for all equipment to be installed with wireless communication devices. The wireless communication device in the era is expected to perform a long time operation using a battery so that it can be used in a place where an infrastructure does not exist, that is, where power cannot be easily supplied. Therefore, a power-saving technique for a wireless communication device is an essential requirement.
A network referred to as a ubiquitous sensor network is intended not only to connect equipments in human's activity area such as various types of equipments for home, but also to connect equipments installed in a place where human cannot live such as a sensor for monitoring a volcano, a desert, a seafloor, etc. Therefore, it is necessary to prepare as many or more wireless communication devices as the equipments installed in home or outdoors, and it is preferable to provide less expensive wireless communication devices.
As a technique of realizing the above-mentioned network, a specification is proposed by, for example, non-patent document 1 (Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks (LR-WPANs) (FIG. 1.), IEEE Computer Society, 804.15.4, Part 15.4). In FIG. 1 of the non-patent document 1, a network is configured by a plurality of equipment units including a wireless communication device, and the data acquired by each equipment unit is transmitted to a server device (PAN coordinator).
Japanese Patent Laid-Open No. 2000-101578 (FIG. 1, FIG. 8) shows a primitive example of a ubiquitous sensor network system in which the operating status of each equipment unit is transmitted to external communication means from a wireless communication device installed in each equipment unit such as a room temperature sensor, an air-conditioner and a hot-water supply system, etc., and the external communication means transfers information about the operational status of each equipment unit to an external communication circuit.
Most of the power supplied to a wireless communication device is used by a radio wave transmitting/receiving unit. A receiving unit is a circuit block for receiving a radio signal having a small voltage amplitude of about several micro-volt (μV) through an antenna, retrieving a signal superposed by a carrier wave, and amplifying a voltage to several voltages at which a digital signal can be processed by current semiconductor LSI technology. On the other hand, a transmitting unit is a circuit block for receiving a digital signal or an analog signal to be transmitted, superposing the signal on a carrier wave, and emitting the result in the air through an antenna.
It is necessary for the transmitting unit to perform transmission using a large amount of power to obtain sufficient reception intensity at a reception point although transmitted radio waves are attenuated in a transmission space, that is, to obtain a sufficiently high signal-to-noise ratio (S/N ratio) at a radio wave reception point. Therefore, the average power consumption of the transmitting unit depends on the operation frequency based on the application field of the ubiquitous sensor network, or depends on the requested distance to be traveled by radio waves or on the S/N ratio. On the other hand, it is necessary that the receiving unit be constantly operated to receive radio waves.
Japanese Patent Laid-Open No. 10-327101 (FIG. 3) and Japanese Patent Laid-Open No. 09-139708 (FIG. 17) show examples of determining the intermittent operation timing between the radio base station or the server device for managing the entire wireless communication system and the wireless communication terminal or the wireless node, and of realizing lower power consumption by the transmitting/receiving units of the wireless communication terminal and the wireless node that are powered by a small battery.
In the ubiquitous sensor network, for example, when it is used for monitoring a natural disaster etc., various sensors and a wireless node for transmitting the information acquired by the sensors are scatteringly arranged in a diameter of several kilometers to several hundreds of kilometers, and the information transmitted from each wireless node is collected by a server device etc.
Since radio waves attenuate in proportion to the square of an achieved distance, it is necessary for each wireless node to transmit radio wave using a large amount of power so that the sever device can receive information from each wireless node arranged over a wide area.
Furthermore, in the networks described in patent documents 2 and 3, it is necessary to transmit radio waves using a large amount of power from a radio base station and a server device in order to synchronize time with a wireless communication terminal arranged in the distance of about several kilometers to several hundreds of kilometers. Therefore, the radio base station and the server device consume a large amount of power.
A configuration can include, for example, a radio wave clock in each wireless communication terminal as a method of synchronizing the time of each wireless communication terminal without transmitting information for time synchronization from a radio base station and a server device. However, when a radio wave clock is included, each wireless communication device is considerably costly. Therefore, it is difficult to apply the concept to the ubiquitous sensor network in which several thousands to several tens of thousands of wireless communication devices are used.
When radio waves are transmitted by using a large amount of power from a radio base station, as in the above-mentioned conventional wireless communication system, and when there is a plurality of wireless communication systems in an arbitrary area, there occurs a problem of interference of radio waves transmitted and received in each wireless communication system. For example, if there is a radio base station that belongs to a wireless communication system, and if there is a wireless communication terminal that belongs to another wireless communication system arranged near the radio base station, then the wireless communication terminal will be strongly influenced by radio waves transmitted from the radio base station. Therefore, it is difficult for the terminal to receive radio waves transmitted from the radio base station of the wireless communication system to which the terminal belongs. In this case, to receive the radio waves transmitted from the radio base station of the wireless communication system to which the terminal belongs, it is necessary to provide a high-performance filter circuit or signal processing circuit to amplify received radio waves by a large gain and to detect a desired signal from the amplified signal. As a result, a wireless communication terminal consumes a large amount of power, and a high-performance circuit is required, thereby producing a costly system.